1. Field of the Invention
This invention pertains generally to electrical switching apparatus and, more particularly, to network protectors. The invention also pertains to a trip coil drive circuit for a network protector.
2. Background Information
Electric power distribution networks, which supply power to a specified area such as, for example, a section of a city, an industrial complex or a large building, are fed at multiple points through feeders, each of which includes a network protector.
A network protector is a circuit breaker adapted to trip and open the feeder upon detection of reverse power flow, that is, power flowing through the feeder out of the network rather than into the network. Typically, overcurrent protection is provided by other devices, such as fuses in series with the network protector.
The network protector is energized by the voltage on the network at the point of connection of the network protector. Standards require that the network protector be able to trip at network voltages as low as 7% of rated voltage. Conventional network protectors have been able to accommodate sufficiently large trip actuator coils having an impedance capable of limiting coil current at the high end of the voltage range while still being able to operate at the low end.
A known network protector trip circuit includes a half-wave rectifier diode and a single capacitor that provides direct current power to the shunt trip coil of the network protector, in order to trip open the same. When either of the rectifier diode or the capacitor fails, the network protector fails to trip open. The failure of the network protector to open, especially during a primary fault, can be destructive to the network protector and to the corresponding feeder transformer, cables and related equipment. For example, the heat generated during such a fault is proportional to the square of the fault current. Such fault current typically runs in magnitudes of thousands of amperes, thereby causing thermal breakdown on all connected equipment or adjacent equipment due to excessive generated heat. In turn, explosion and fire can cause destruction and excessive damage.
There is a need, therefore, for an improved, increased reliability network protector and trip coil drive circuit.
Accordingly, there is room for improvement in electrical switching apparatus and trip coil drive circuits for electrical switching apparatus.